Press bending of glass sheets

ABSTRACT

A glass sheet is bent to a required curvature between two relatively movable complementary bending dies which deform the glass sheet, which is heated to a temperature at which it is pliable. The respective overall heat losses from the two major surfaces of the glass sheet are adjusted during the bending operation so as to be substantially equal. For example, heat loss to one die may take place predominantly by conduction and the temperature of this die may be controlled, by means of heaters, to regulate the conductive heat loss associated with this die and effect the required heat loss balance.

[451 Aug. 21, 1973 PRESS BENDING OF GLASS SHEETS [75] inventors: JohnPickard, Studley; Thomas Cartledge, Teesside, both of England [73]Assignee: Triplex Safety Glass Company Limited, London, England [22]Filed: July 2, 1969 [2i] Appl. No.: 838,612

[30] Foreign Application Priority Data 1/1969 Kirkman 65/273 PrimaryExaminer-Arthur I). Kellogg Attorney-imirie, Smiley, Snyder and Butrum[57] ABSTRACT A glass sheet is bent to a required curvature between tworelatively movable complementary bending dies which deform the glasssheet, which is heated to a temperature at which it is pliable. Therespective overall heat losses from the two major surfaces of the glasssheet are adjusted during the bending operation so as to besubstantially equal. For example, heat loss to one die may take placepredominantly by conduction and the temperature of this die may becontrolled, by means of heaters, to regulate the conductive heat lossassociated with this die and effect the required heat loss balance.

7 Claims, 2 Drawing Figures Patented Aug. 21, 1 973 I I I7 Inventor; JoHN P: 6 KA R D TH on As CARTLEDGE B1 /WL Attorneys PRESS BENDING OF GLASSSHEETS BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to the bending of glass sheets to a requiredcurvature.

2. Description of the Prior Art One known method of bending a glasssheet is to place the sheet, usually suspended vertically, between apair of complementary bending dies, the sheet being heated to atemperature at or above the temperature at which the sheet can be formedto the desired shape. Relative movement, usually horizontally, of thetwo dies towards each other deforms the glass sheet to the curvature ofthe dies.

It is well-known that, when bending a glass sheet in this way, a furtherchange in the shape of the curved glass sheet occurs after the bendingdies have been retracted. This phenomenon, often referred to as springback results from the differential cooling of the two major surfaces ofthe curved glass sheet which occurs when the sheet is deformed betweenthe complementary bending dies, particularly if these dies are ofdissimilar construction. When the dies are subsequently retracted, atemperature difference therefore exists between these two majorsurfaces, and as this temperature difference decays it causes distortionof the glass sheet.

An object of the present invention is to minimise the effect of springback and thereby to permit bending of glass sheets with a consistentaccuracy.

SUMMARY The present invention accordingly provides a method of bending aglass sheet to a required curvature in which the sheet is heated to atemperature at which bending can be effected and is disposed between tworelatively movable complementary bending dies which deform the heatedglass sheet to a desired curvature, characterised in that the respectiveoverall heat losses from the two major surfaces of the glass sheetduring the bending operation are adjusted so as to be substantiallyequal.

The balance between the overall heat losses from the respective majorsurfaces of the glass sheet may be effected by controlling the heat lossfrom one of the major surfaces during the bending operation. This isconveniently effected by arranging that the heat loss from said onemajor surface occurs predominantly by conduction to the respectivebending die, and by regulating the conductive heat loss associated withsaid die by controlling the temperature of said die. Thus said die maybe heated in a controlled manner so as to maintain its bending surfaceat a predetermined temperature lower than the temperature of the glasssheet as it is bent.

The bending temperature of industrial soda-limesilicate glass sheetslies between 550 and 700C, and can, for example, be about 670C. Forbending such glass sheets by the method of the present invention thesaid bending die to which heat loss occurs predominantly by conductionis preferably maintained at a surface temperature of 200 to 400C, atemperature of about 300C being preferred.

The present invention also provides apparatus for use in bending a glasssheet to a required curvature, comprising two relatively movable bendingdies having complementary bending surfaces, means for supporting a glasssheet between the dies, the glass sheet being heated to a temperature atwhich bending can be effected, and means for moving the dies relativelytowards each other to effect bending of the glass sheet, characterisedin that one only of the dies is so constructed that heat loss from theglass sheet to the one said die takes place primarily by conduction andtemperature-regulating means are incorporated in said one die forregulating the temperature thereof in order to control the overall heatloss to said one die during the bending operation, so as substantiallyto balance the overall heat loss to said one die with the overall heatloss to the other said die.

The said one die, which in a preferred embodiment of the invention has aconvex bending surface, may be of solid construction, having containedtherein at least one electrical heating element and at least onetemperature-sensing element disposed near the bending surface of the diefor monitoring the temperature of said surface.

Preferably, a plurality of electrical heating elements and associatedtemperature-sensing elements are provided in said one die, said elementsbeing arranged in respective zones, each provided with a respectiveregulating device for regulating the electric current supplied to therespective heating element or elements in said zone. The respectivecurrent-regulating devices may comprise respective variabletransformers.

The heat-conductive die may alternatively comprise a curved metal platehaving radiant heaters for example gas or electric heaters arrangedbehind the plate.

The bending surface of the said one die may be covered with a layer of amaterial which is non-sticking or non-wettable with respect to glass.One such material is boron nitride, but glass fibre cloth is preferredin practice.

The invention also comprehends a glass sheet which has been bent by themethod herein defined.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic perspectiveview, by way of example, of apparatus for use in bending glass sheetsaccording to the invention, and

FIG. 2 is a cross section, taken in a vertical plane, through the convexor male bending die of said apparatus, and showing diagrammatically theelectrical connections to one of the heated zones thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FlG. l, a flatglass sheet I to be bent is suspended in a vertical plane by tongs 2which grip the upper edge of the sheet 1. The tongs 2 are supported by ahorizontal overhead conveyor 3, part only of which is shown.

The glass sheet 1, suspended by the tongs 2, is first passed by theconveyor 3 through a furnace (not shown) in which the sheet is heated toa temperature, of the order of 670C for industrial soda-lime-silicateglass, at which the sheet is sufficiently pliable to permit bending to adesired shape.

The heated glass sheet 1 is passed from the furnace to a bending stationat which the sheet is disposed between two bending dies 4, 5 which aremounted on suitable supports (not shown in the interests of clarity) formovement horizontally towards and away from each other as required. Thedies 4, are complementary, the die 4 in this case being a convex or maledie and the die 5 being a concave or female die.

The convex die 4 in the illustrated embodiment is of solid construction,and is formed from a cast refractory material. A suitable refractorymaterial comprises four parts of fused silica crushed to pass through alO-mesh sieve, mixed with one part of Ciment Fondu which is a hydraulichigh alumina cement and water: such a mixture can be moulded easily togive a convex bending surface of the required curvature. For somebending operations, for example, for forming curved glass windscreens orrear lights of motor vehicles, the bending surface may have doublecurvature, for example double convex curvature as in the illustratedembodiment.

Upon casting of the refractory convex die 4 a plurality of electricalheating elements 6 are embedded in the die, the elements 6 being showndiagrammatically in broken lines in FIG. 1. The refractory material ofthe die 4 is packed tightly around the elements 6 upon moulding to givegood thermal contact.

The heating elements 6 can conveniently be formed by winding 18 S.W.G.This wire is nickel-chrome alloy with the addition of small percentagesof other metals to achieve a long lasting heating element. Brightray(Registered Trade Mark) C wire upon a ,4" diameter mandrel. A typicalheating element consists of a 62 ft. length of wire having a resistanceof 17.7 ohms and taking 13 amps from a 230 volt supply. Such a heatingelement gives a heat output of about 26 watts per square inch.

To facilitate precise control of the surface temperature of the convexbending die 4, the die is divided into a number of separate controlzones, six such zones being shown in this example. The number of heatingelements 6 associated with each zone is selected in accordance with thesize of the zone, the heating elements of any one zone being connectedin parallel and connected by internal leads (not shown) to a respectivepair of porcelain-insulated terminals.

Respective temperature-sensing elements, in this case thermocouples, arealso embedded in the convex die 4, as indicated in broken lines at 7 inthe drawings, one thermocouple 7 being provided for each control zone,with the temperature-responsive junction close to the convex bendingsurface of the die 4 and centrally in the respective zone. The leadsfrom each thermocouple 7 are connected to further respective pairs ofporcelaininsulated terminals.

The terminals associated with the heating elements 6 and thermocouple 7of each respective control zone are connected by respectiveasbestos-insulated cables to respective terminals of a junction boxdisposed at the periphery of the die 4, and shielded from exposure toheat from said die. A remote control unit having separate controlsections for each respective control zone of the die 4, is connected tothis junction box. The control unit, part of which is indicateddiagrammatically by the broken lines 8 in FIG. 2, includes, for eachrespective control zone, a respective variable transformer 10 forcontrolling the power supplied from an alternating current supply to therespective heating elements 6 of said zone and a respectivemicro-voltmeter l l for measuring the electro-motive force generated inthe respective thermocouple 7 of said zone. Preferably themicrovoltmeters 11 are calibrated to give direct readings of thetemperature inthe respective zones. The variable transformers 10 can beadjusted manually in order to maintain a predetermined uniformtemperature, for example, 300C in each zone, or, alternatively, anautomatic control system may be provided for maintaining saidpredetermined temperature automatically.

The concave or female die 5 comprises a metal open frame structurehaving shaped peripheral frame members 12 which are adapted to engagethe marginal regions of the glass sheet 1 when effecting bending. Tominimise marring of the surface of the glass sheet the frame members 12may be covered with woven glass cloth or some other material to whichglass does not stick.

The bending of each glass sheet 1 to the desired shape takes place bycontact of the sheet with the bending surfaces of the complementary dies4, 5 when the latter are advanced towards each other. Typically, thedies 4, 5 are held in their closed position for about two seconds.

During this time when the dies 4, 5 are in their closed position, heatloss occurs from the two major surfaces of the glass sheet 1. The convexsurface of the sheet 1, in contact with the skeletal female die 5, losesheat by radiation and convection, and the concave surface, in contactwith the solid convex or male die 4, loses heat mainly by conduction.

It has been found that the phenomenon of springback, that is, thedistortion of the curved glass sheet after completion of the bendingoperation, is due almost entirely to the differential heat losses whichoccur from the two major surfaces of the glass sheet. In appreciation ofthis, the present invention is seen to provide means whereby the heatloss from one of the major surfaces, specifically the heat loss byconduction from the concave surface in contact with the convex die 4,can be regulated. Thus the heating elements 6 in the convex die 4 arecontrolled so as to limit the conductive heat transfer from the sheet Ito the die 4, so maintaining the temperature of the working surface ofthe die 4, that the overall heat loss from the glass surface in contactwith the die 4 substantially balances the overall heat loss from theother surface engaged by the die 5.

In the example described, if the convex die 4 is maintained at atemperature between 200 and 400C, and preferably about 300C, the desiredthermal balance between the two dies 4, 5 can be achieved, for a dieclosure time of about 2 seconds. After the curved sheet 1 has beenremoved from between the dies 4, 5 and allowed to cool, the spring-back"of the curved sheet is much reduced, and hence the curvature of thesheet ultimately obtained has a much closer conformity to theconfiguration of the surface of the convex bending die 4 than it wouldotherwise have.

As an alternative to using cast refractory material for the convex die 4a cast metal convex die or a metal die formed conventionally from metalplate or sheet may be used. A satisfactory metal for the die 4 is analuminum or a copper based alloy in cast form for example, an alloycontaining 1 percent chromium and 99 percent copper. Such a metal diehas the advantage of high strength combined with high conductivity, thelatter assisting in accurately maintaining a controlled temperature atthe surface of the die 4. Control of the temperature of the die surfacemay be effected by means of radiant gas or electric heaters locatedbehind the die.

Specifically, where a cast metal die is used, the surface temperaturemay be regulated by means of electrically insulated high temperatureheaters of the PYROTE- NAX (Registered Trade Mark) type inserted insuitable holes in the rear of the die. These heaters are formed byencasing a nickel-chrome heating element in a high temperature mineralinsulation such as magnesium oxide within a metal sheath formed ofnichrome.

If a cast metal is used to form the convex die 4, the die surface ispreferably machined and subsequently hand polished to give a fine,smooth finish.

Where a solid cast metal convex or male die 4 is employed, it is nolonger necessary to embed heating elements within the body of the die,as in the embodiment specifically illustrated: in view of the highconductivity of a solid metal die it is satisfactory to provide dieheaters in the form of heating elements clamped to the rear surface ofthe die. Suitable strip heating elements are those of the Chromalox(Trade Mark) type, consisting of a nickle-chrome heating conductorcontained within a ceramic filled nickel alloy sheath.

The bending surface of the convex or male die 4 in the embodimentsherein described in preferably covered with a layer of a material whichis non-sticking or non-wettable with respect to glass, in order tominimise marring of the glass surface upon contact with the die. Asstated previously, a suitable such material is woven glass fibre cloth.Boron nitride may alternatively be used, being applied to the metal diesurface by spraya ing, brushing or dipping.

We claim:

1. Method of bending a glass sheet to a required curvature, comprisingheating the glass sheet to a temperature within the bending temperaturerange of the glass, locating the heated glass sheet between a firstbending die of open form for contacting a minor portion only of onesurface of the sheet and an opposed complementary second bending die forcontacting a major portion of the other surface of the sheet, moving thedies relatively towards each other to bend the heated glass sheet to therequired curvature, and heating the second bending die to adjust therate of heat loss from said other surface of the sheet substantially tothe rate of heat loss from said one surface of the glass sheet duringthe bending operation to effect a balance between the respective overallheat losses from the two surfaces of the glass sheet.

2. Method according to claim 1, in which the heating of the secondbending die is continuously controlled throughout the bending operationto effect the balance between the respective overall heat losses fromthe two surfaces of the glass sheet.

3. Method according to claim 2, in which the heat loss from said onesurface occurs predominantly by conduction to the second bending die,and in which the heating of said second bending die is controlled toregulate the conductive heat loss associated with said sec ond bendingdie.

4. Method according to claim 3, in which said second bending die isheated in a controlled manner to maintain its bending surface at apredetermined temperature lower than the temperature of the glass sheetas it is bent.

5. Method according to claim 4, in which said predetermined temperaturelies in the range 200 to 400C for a die closure time of about 2 seconds.

6. Method according to claim 5, in which said predetermined temperatureis of the order of 300C.

7. The method of forming a glass sheet to provide respectively concaveand convex opposite surfaces thereon having selected curvatures, whichcomprises the steps of:

a. heating the sheet while flat to a selected temperature within thebending temperature range of the glass;

b. contacting a minor portion only of one surface of the sheet while atsaid selected temperature with a concave bending die whereby heattransfer from said one surface occurs at a particular rate due in partonly to contact with said concave bending die;

c. simultaneously with (b), contacting at least a major portion of theother surface of said sheet with a convex bending die whereby heattransfer from said other surface occurs at least predominantly throughheat loss to said convex bending die;

d. controlling the temperature of said convex bending die to adjust therate of heat transfer from said other surface of said sheetsubstantially to said particular rate at which heat is transferred fromsaid i one surface of said-sheet whereby to obtain substantial equalityof temperatures for the convex and concave surfaces of said sheet formedrespectively by steps (b) and (c); and

e. removing said bending dies from contact with said sheet while thesubstantial equality of temperatures specified in step (d) prevailswhereby said convex and concave surfaces of the sheet retain saidselected curvatures thereof as imparted by said bending dies.

1. Method of bending a glass sheet to a required curvature, comprisingheating the glass sheet to a temperature within the bending temperaturerange of the glass, locating the heated glass sheet between a firstbending die of open form for contacting a minor portion only of onesurface of the sheet and an opposed complementary second bending die forcontacting a major portion of the other surface of the sheet, moving thedies relatively towards each other to bend the heated glass sheet to therequired curvature, and heating the second bending die to adjust therate of heat loss from said other surface of the sheet substantially tothe rate of heat loss from said one surface of the glass sheet duringthe bending operation to effect a balance between the respective overallheat losses from the two surfaces of the glass sheet.
 2. Methodaccording to claim 1, in which the heating of the second bending die iscontinuously controlled throughout the bending operation to effect thebalance between the respective overall heat losses from the two surfacesof the glass sheet.
 3. Method according to claim 2, in which the heatloss from said one surface occurs predominantly by conduction to thesecond bending die, and in which the heating of said second bending dieis controlled to regulate the conductive heat loss associated with saidsecond bending die.
 4. Method according to claim 3, in which said secondbending die is heated in a controlled manner to maintain its bendingsurface at a predetermined temperature lower than the temperature of theglass sheet as it is bent.
 5. Method according to claim 4, in which saidpredetermined temperature lies in the range 200* to 400*C for a dieclosure time of about 2 seconds.
 6. Method according to claim 5, inwhich said predetermined temperature is of the order of 300*C.
 7. Themethod of forming a glass sheet to provide respectively concave andconvex opposite surfaces thereon having selected curvatures, whichcomprises the steps of: a. heating the sheet while flat to a selectedtemperature within the bending temperature range of the glass; b.contacting a minor portion only of one surface of the sheet while atsaid selected temperature with a concave bending die whereby heattransfer from said one surface occurs at a particular rate due in partonly to contact with said concave bending die; c. simultaneously with(b), contacting at least a major portion of the other surface of saidsheet with a convex bending die whereby heat transfer from said othersurface occurs at least predominantly through heat loss to said convexbending die; d. controlling the temperature of said convex bending dieto adjust the rate of heat transfer from said other surface of saidsheet substantially to said particular rate at which heat is transferredfrom said one surface of said sheet whereby to obtain substantialequality of temperatures for the convex and concave surfaces of saidsheet formed respectively by steps (b) and (c); and e. removing saidbending dies from contact with said sheet while the substantial equalityof temperatures specified in step (d) prevails whereby said convex andconcave surfaces of the sheet retain said selected curvatures thereof asimparted by said bending dies.